Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

Chapter 3 / Sensors
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As with time delays, the gain and phase errors plotted against normalised
frequency in Figure 3-14 and Figure 3-15 must be closely monitored. ADC
scale factors and biases are usually small and are ignored.
GAIN ERROR (DB)
GAIN ERROR (DB)
0
-0.2
-0.4
-0.6
-0.8
-1
-1.2
-1.4
-1.6
-1.8
-2
0 0.1 0.2 0.3 0.4 0.5
NORMALISED FREQUENCY
10
9
8
7
6
5
4
3
2
1
Figure 3-14 : ZOH Gain Error
( 1 st order – solid ; 2 nd order – dashed )
0
0 0.1 0.2 0.3 0.4 0.5
NORMALISED FREQUENCY
Figure 3-15 : ZOH Phase Error
( 1 st order – solid ; 2 nd order – dashed )
Consider a unit amplitude, 1 Hz signal passing through a 4 th order
Butterworth A/A filter with a 20 Hz bandwidth. A 40 Hz ZOH “freezes”
the input whilst the 12 bit ADC output is raised to within ± 0.5*LSB of the
input.
By design, the ADC must be capable of settling within the ZOH time if the
internal measure passed to the output buffer is to be the correct value, in
which case their internal high frequency dynamics can be ignored.
3.2-24